Actually, in 1915 the Supreme Court decided that they are legal. And further, the granting of a pardon imputes guilt on the recipient, and the recipient must accept it for it to be valid. And even further, if the recipient accepts it, they do not have any 5th Amendment protection to refuse to testify.
See: https://supreme.justia.com/cases/federal/us/236/79/
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Right Burdixk v United States and United States v Wilson...total of 7 cases...thing is they testify all the ones who were also involved get outed...